In recent years, increase in plastic waste has become a great social problem. Since many of polymeric materials have hitherto been developed and produced in search of high performance and long-term stability, they are not easily decomposed in a natural environment. Therefore, how to dispose and manage a large quantity of plastic waste which has become useless becomes a social problem on a world-wide scale. The plastic waste includes sheets formed from a variety of synthetic resins, such as polyolefin resins such as polyethylene and polypropylene, polyester resins such as polyethylene terephthalate, polyamide resins such as nylon, and chlorine-containing resins such as polyvinylidene chloride, and trays and containers obtained by secondarily processing these sheets.
Under the circumstances, biodegradable polymers, which are degraded by natural microorganisms, attract attention as polymeric materials which impose no burden on the environment. The biodegradability can be evaluated by, for example, a degradability test in soil (soil degradability test). Since plastic sheets are required to have good mechanical properties, thermal properties, barrier properties, melt processability, profitability and the like, however, any plastic sheet, which fully satisfies these requirements and exhibits biodegradability, has not been yet obtained.
Among the conventional biodegradable plastic sheets, for example, sheets based on starch are unsatisfactory in toughness, barrier properties and heat resistance and involve a problem that they are difficult to melt-process, so that its processing cost becomes high. Sheets based on cellulose are unsatisfactory in toughness and involve a problem that they are difficult to melt-process, so that its processing cost becomes high. Sheets based on a microorganism-produced polyester are unsatisfactory in toughness and involve a great problem that their production cost becomes particularly high. Sheets based on a synthetic type polyester such as a polysuccinate are unsatisfactory in toughness and barrier properties and involve a problem that succinic acid and butanediol, which are raw materials for the polyester, are considerably expensive.
Sheets based on polylactic acid, which is a semi-synthetic type polyester, are unsatisfactory in toughness. Since L-lactic acid, which is an optically active substance used as a raw material, is required to have a high purity, the sheets must be produced through fermentation of a biological process, and there is hence a limit to their production at low cost. Further, since polylactic acid has a high glass transition temperature, Tg, it also involves a problem that it is difficult to compost under ordinary composting conditions.
With respect to films based on polyglycolic acid, which are synthetic polyester films, Higgins et al. (U.S. Pat. No. 2,676,945, issued in 1954) disclose amorphous films having a thickness of 3 mils and biaxially stretched films obtained by stretching them. These films are those obtained by using a polymer obtainable by directly polycondensing glycolic acid, not a polymer obtainable by ring-opening polymerization of glycolide. However, the polycondensation process of glycolic acid includes heating and polycondensing glycolic acid for a long period of time as about 40 hours at a high temperature of at least 200.degree. C. and hence tends to involve side reactions such as decomposition reaction. Accordingly, this process is difficult to provide a practicable polymer having a high molecular weight. U.S. Pat. No. 2,676,945 describes, in its example, the melt viscosity of the polymer as about 2,000 poises (at 245.degree. C.). This melt viscosity value corresponds to a value measured at a shear rate of about 0/sec. This melt viscosity value corresponds to a value extremely as low as about 20 poises (at 245.degree. C.) when converted into a value measured at a shear rate of 100/sec. In addition, there is a high possibility that this polymer may have an unstable structure due to side reactions. Accordingly, an extrusion film formed from the polymer obtained by such a direct polycondensation process has extremely low mechanical strength as demonstrated by its tensile strength extremely as low as 5,470 psi (about 39 MPa) and hence has involved a problem that it is unsatisfactory from the view point of practical use.
Gilding et al. POLYMER, 20, 1459 (1979)! provide films having a thickness of 250 .mu.m from 20% solutions of glycolic acid copolymers (glycolide/lactide=90/10, 70/30, 50/50, etc.) by a casting process. However, this process tends to form coarse spherulites upon evaporation of a solvent, and the resultant films are hence extremely brittle and unsatisfactory in mechanical strength from the viewpoint of practical use.
Japanese Patent Application Laid-Open Nos. 256480/1994 and 256481/1994 disclose that polyglycolic acid having a viscosity-average molecular weight of 280,000 or 250,000 was used as a raw material, this polymer was melt-extruded and cast at 280.degree. C. into an unoriented sheet, and the unoriented sheet was then stretched 3 times in a machine direction and 4 or 3 times in a transverse direction at 160.degree. C. to obtain a biaxially oriented film (in each document, Example 3). However, polyglycolic acid tends to undergo thermal decomposition at a temperature exceeding about 255.degree. C. and actually substantially undergoes thermal decomposition at the temperature as high as 280.degree. C., so that any satisfactory unoriented sheet cannot be obtained. It is a matter of course that if such an unoriented sheet is biaxially stretched, it is scarcely possible to obtain a biaxially oriented film having sufficient strength. An amorphous sheet of polyglycolic acid is crystallized at a temperature exceeding its crystallization temperature Tc.sub.1 (about 80.degree. C.) and highly crystallized at a temperature as high as 160.degree. C., so that the biaxial stretching of the amorphous sheet, for example, 3 times in a machine direction and 3 or 4 times in a transverse direction, is extremely difficult or actually impossible under such stretching conditions. Accordingly, any sheet or oriented film having sufficient strength cannot be obtained under the conditions disclosed in these publications. Besides, the meaning of the viscosity-average molecular weight of 250,000 or 280,000 and its measuring method are also unclear. Accordingly, it can be hardly said that these publications actually disclose high-molecular weight polyglycolic acid in the light of the state of the art.